Zoned lens

ABSTRACT

A zoned lens comprises at least two adjacent zones (16) formed such that the differences between the optical path lengths between an object point and a pixel of light beams extending through these two adjacent zones (16) of the lens (15) are at least equal to half the coherence length of the light used, preferably at least equal to the coherence length of the light used.

FIELD OF THE INVENTION

The invention relates to a zoned lens.

BACKGROUND OF THE INVENTION

Lenses are in this case generally understood to be both refractivelenses and diffractive lenses (for example, Fresnel zone plates). Adiaphragm provided with a small hole can also be provided with effectivepowers (effective refractive powers), as described with reference to thedrawings. Optical elements with a zero nominal power but with a limitedaperture such as, for example, circular or annular plates with parallelfront and rear surfaces can therefore also be understood here as"lenses".

Different zones of a lens are represented by areas of a lens which canbe differentiated by physical properties, wherein between the zones thephysical properties alter very rapidly or abruptly over a relativelysmall area. In particular, step-like sudden changes in the thickness ofthe lens material being used, for example, steps on the front or rearsurface of the lens, delimit different zones of a lens. The use ofdifferent lens materials in different areas of the lens also results ina zoned lens in the present sense. The geometrical arrangement of thedifferent zones can be configured in many different ways. For example, acentral circular zone can be provided, to which outer concentric annularzones are adjacent. The surface areas of the different zones can be thesame or different.

SUMMARY OF THE INVENTION

The object of the invention is to provide a zoned lens with noveloptical properties. This is achieved according to the invention in thatit comprises at least two adjacent zones, wherein the differences in theoptical path lengths between an object point and an image point, oflight rays passing through these two adjacent zones of the lens, are atleast equal to half the coherence length of the light used, preferablyat least equal to the coherence length of the light used. If there areoptical materials with different refractive indices in the differentzones, in such a configuration it can advantageously be provided thatany two rays parallel to the axis through two adjacent zones passthrough optical path lengths within the lens which differ by at leastthe coherence length. If adjacent zones are made from the same lensmaterial, the optical path lengths of any two rays parallel to the axisthrough the two adjacent zones have to differ by at least CL n_(c)/(n_(c) -n_(i)), wherein CL is the coherence length of the light used,n_(c) is the refractive index of the lens material and n_(i) is therefractive index of the medium surrounding the lens. The values n_(c)=1.5 and n_(i) =1 produce, for example, 3 CL. This condition issatisfied when, for example, steps of a height resulting from CL/(n_(c)-n_(i)) are provided between the zones. The values selected for n_(c)and n_(i) and the coherence length of, for example, 2 micrometers,produces a step height of 4 micrometers.

In the case where the differences in the optical path lengths of thelight rays which pass through adjacent zones of the lens between anobject point and an image point are at least equal to the coherencelength of the light used, there is no interference of the light rayspassing through the adjacent zones, as will be described in more detailwith reference to the drawings, whereby in various applicationsadvantages can be obtained compared to conventional lenses. For example,in bifocal lenses disturbing interferences between the areas withdifferent focal lengths can be eliminated.

If the steps are configured so that the optical path length differencesof the light rays through adjacent zones between an object point and animage point are shorter than the coherence length but at least equal tohalf the coherence length of the light used, the interference of thelight rays passing through adjacent zones is reduced but not completelysuppressed, which may be sufficient for some applications. Zoned lenseswith such reduced steps are thus also the subject-matter of theinvention.

In an embodiment of the invention it is provided that adjacent zones ofthe lens have a different thickness, wherein steps are provided betweenthe zones. The step height between adjacent zones of the lens has to beat least |λ² /(Δλ(n_(c) -n_(i)))|, wherein λ is the average wavelengthof the light used, Δλ is the half-value width of the wavelengthdistribution of the light used, n_(c) is the refractive index of thelens material, and n_(i) is the refractive index of the medium adjacentto the lens. The coherence length is given by CL=λ² /Δλ. When visiblelight with a coherence length in the region of approximately 1-10 μm isused, the step height, measured in micrometers, between adjacent zonesof the lens has to be at least 5/|(n_(c) -n_(i))|, wherein n_(c) is therefractive index of the lens material and n_(i) is the refractive indexof the medium adjacent to the lens. Advantageously, a step height of atleast 3 micrometers, preferably of at least 10 micrometers, is provided.As the standard lens materials have a refractive index of approximately1.5, and as the light reemitted from subjects illuminated with "white"light has coherence lengths which are normally in the region of 3 μm,however seldom above 5 μm, interference between the different zones canbe effectively eliminated with such a step height.

Such a zoned lens according to the invention can be configured, forexample, such that a central circular zone is provided and around it areadjoining concentric annular zones, wherein the surface areas of all thezones is the same. Such a zoned lens can satisfy the otherwisecontradicting requirements of having a wide distribution of theeffective power and a high intensity of light allowed to pass through,as described in more detail with reference to the drawings. In order toeliminate scattered light from the lateral step surfaces between thezones, in this embodiment of the invention it is advantageously providedthat the lateral surfaces of the steps between the zones are providedwith a light absorbing material.

The optical path length of a light ray from an object point through thelens to a conjugated image point belonging to this object point iscomposed of the two values outside the lens and the value within thelens. With the precautions described above, the optical path lengths oflight rays through two adjacent zones differ by at least the coherencelength. If--as in the case of multifocal lenses--there are several imagepoints conjugated to one object point, the optical path lengths throughadjacent zones have to measured between the object point and always thesame image point. Any of the available conjugated image points, whichmay be real or virtual, can be used for this purpose.

The determination of a conjugated image point belonging to a specificobject point is known prior art. Examples of methods for implementingthis are, however, briefly described again in the description of thedrawings.

Further advantages and details of the invention will be described in thefollowing with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These show, in

FIGS. 1a and 1b, schematic representations of the ray angle of radiatingsurfaces,

FIG. 2 a distribution of the effective power of a lens,

FIG. 3 the dependency of the half-value width of the distribution of theeffective power upon the lens aperture,

FIG. 4 the dependency of the half-value width of the effective power,normalised to the wavelength of the light, upon the lens surface,

FIG. 5 distributions of effective powers with different lens apertures,

FIG. 6 a comparison of the interference pattern of a circular surfacewith an annular surface with the same surface area,

FIG. 7 reflectance spectra of difference items,

FIG. 8a an embodiment of the invention,

FIG. 8b a second embodiment of the invention,

FIG. 9 a third embodiment of the invention,

FIG. 10 a contact lens according to the invention,

FIG. 11 an auxiliary drawing showing the construction of a lens,

FIG. 12 the distribution of the effective powers of a lens according tothe invention compared to a conventional lens with a surface ten timessmaller,

FIGS. 13a and 13b, distributions of the effective powers of bifocallenses without and with interference between the zones,

FIG. 14 a distribution of the effective powers of a lens made frombirefringent material,

FIG. 15 a further embodiment of the invention,

FIG. 16 a comparison of the effective power of a conventional lens withthat of a system composed of a conventional lens and a lens according tothe invention,

FIGS. 17 to 20, further embodiments of the invention,

FIG. 21 a comparison of the effective powers of different refractivebifocal zoned lenses, both conventional and according to the invention,

FIG. 22 the dependency of the optical path lengths upon the distanceapart of the light rays from the centre point of the lens for the imagepoint for a power of -5 dioptres,

FIG. 23 a further embodiment of the invention,

FIG. 24 the dependency of the optical path length differences upon thedistance of the rays from the central point of the lens in the case of aconventional refractive bifocal zoned lens,

FIG. 25 the dependency of the optical path length upon the distance ofthe rays from the central point of the lens of a bifocal zoned lensaccording to the invention at an effective power of 3.75 dioptres.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is known that the ray width of the light from a radiating surface isgreater the smaller the radiating surface. This physical fact can beexplained by examination of interference: FIG. 1a shows in a schematicmanner a small surface 1, which emits, for example, coherent light 2,wherein all the partial waves transmitted may have identical phases atthe location of the emission. The ray width α_(A) at which the initiallydestructive interference between the marginal rays or waves occurs, isgreater than the width α_(B) with the same conditions as in FIG. 1b. Itcan be directly concluded from this that, for example, a lens with asmaller aperture has a greater depth of field than a lens with a largeraperture.

A practical application of this fact is represented by so-calledstenopaic spectacles: According, for example, to Graefe-Saemisch:Handbuch der gesamten Augenheilkunde [Handbook of General OpticalMedicine], Wilhelm Engelmann publications, Leipzig 1910, page 178,stenopaic spectacles are "characterised by those devices wherein agreater focus of the retinal image (comment: optically refractivedevices) is not caused by changing the ray path, but instead in thatattempts are made to reduce the dimension of the circle of divergence bysuitable apertures, that is to say by the limitation of the incident raycluster. The simplest form of such spectacles is a diaphragm providedwith a very small hole".

The disadvantages of such stenopaic spectacles are essentially apparentin the reduction of the light intensity and in the restriction of thefield of vision; further, the capacity for resolution and contrast ofoptical devices generally reduces with increasing depth of field. Withdifferent optical applications, the reduction of the contrast andresolution is acceptable, however.

The disadvantage of the narrowed field of vision can be alleviated bybringing the hole closer to the eye, that is to say by configuring the"spectacle" as a contact lens (or as an intra-ocular lens). Thedisadvantage of the reduced light intensity is occasionally countered byhaving several adjacent holes (see Graefe-Saemisch, loc.cit). Withadjacent holes, however, interference is produced between the lightwaves from the different holes, which disadvantageously affects theimaging quality.

With the present invention, a radiating surface can be designed so thateach individual partial area of this surface emits light practicallyunaffected by the other partial areas, so that the interference patternsbehind the individual partial areas exist independently of one another,and consequently do not interfere with the wave trains coming fromdifferent partial areas. In this way, for example, the light intensityof stenopaic spectacles or lenses can be increased substantially withoutreducing the optical advantages produced by the restriction of theincident light ray clusters to very small apertures. Furthermore, withthe present invention, lenses with a relatively large aperture can bemanufactured which have the depth of field of lenses with a very smallaperture, which however allow substantially greater light intensities topass through than the corresponding lenses with a small aperture.Further, with the present invention devices can be produced with whichconventional lenses with a large aperture can be given the opticalcharacteristics of lenses with small apertures. Such "stenopaic lenses"with a large aperture and light intensity can be used in many ways inoptical apparatuses and optical and ophthalmic devices. For example, inthis way ophthalmic viewing aids can be manufactured which serve tocorrect geriatric vision and/or for the correction of astigmatism.Further, it is also possible with the present invention to manufacturebifocal or multifocal zoned lenses in which interference between theindividual zones is suppressed. It is also possible to manufacturebifocal or multifocal lenses in which only zones with the same nominalpower can interfere. Lastly, it is possible with the present inventionto manufacture Fresnel zone plates which have substantially double thelight yield of conventional Fresnel zone plates.

FIG. 2 shows the distribution of the effective power of a lens of 1 mmdiameter and nominally 4 dioptres, that is to say the intensity of thelight 2 is plotted with respect to the effective power D_(eff). As canbe seen, because of this small aperture of 1 mm, this lens provides awide spectrum of effective power. As a measurement of the width of thedistribution of the power, the half-value width ΔP of the distributionis assumed; for the lens evaluated in FIG. 2, the half-value width (fora light wave length of 560 mm) is approximately 4 dioptres.

The calculation of the distribution of the effective power of a lens--asshown, for example, in FIG. 2--can be done in different ways. Twomethods for this are described briefly hereafter:

Method 1:

j connecting rays from an object point O located on the axis of the lensg meters in front of the front surface of the lens are drawn to j pointsevenly distributed on the front surface of the lens. Then, from each ofthe j points on the front surface k connecting lines are drawn to kpoints evenly distributed on the rear surface of the lens. (Withnumerical evaluation, it makes sense to place the j and k points in anorthogonal grid). Then the k points on the rear surface are connected toan image point B located on the axis of the lens b meters behind thelens (g and b can also have negative values).

Thus there are altogether j*k wave trains which connect the object pointO to the image point B. The optical path lengths L_(jk) between O and Bare then determined for each of these j*k wave trains. The resultingamplitude in the image point B then produces

    A.sub.res =Const.(Σ sin φ.sub.jk +Σ cos φ.sub.jk); φ.sub.jk =(L.sub.jk /λ)2π,

wherein λ is the wavelength of the light. The summation has to beextended to all the j's and k's. The intensity resulting in B is thenA_(res) ².

The associated "effective power" D_(eff) is further given by

    D.sub.eff =1/g+1/b

Distribution curves as shown in FIG. 2, are obtained by changing g or b;the two values g and b can also be varied simultaneously.

Method 2:

As in method 1, an object point O is connected to j evenly distributedpoints on the front surface of the lens. These connecting lines thenrepresent j light rays, for which the law of refraction is used toobtain the j broken light rays within the lens. The broken j light raysmeet in j points on the rear surface of the lens; now each of these jpoints is connected to the point B. In this way j wave trains areobtained between the points O and B and consequently Lj differentoptical path lengths of the j wave trains. The resulting amplitude in Bis then given by

    A'.sub.res =Const*(Σ sin φ.sub.j +Σ cos φ.sub.j) with φ.sub.j =(L.sub.j /λ)*2π

The summation now has to be extended to all j wave trains.

The two methods give practically the same result, when summation takesplace over sufficient rays and points. As the complexity of thecalculation is substantially less with method 2, this method is to bepreferred. The characterisation of lenses set out hereafter is also doneaccording to the principles of method 2. To calculate differenteffective powers D_(eff), the image distance b can further be keptconstant and only the object distance g varied, whereby the 1/r-loss ofthe amplitude does not have to be taken into account explicitly.

According to the formula set out above for the effective power D_(eff),it should be noted that a given object width g_(k) is connected to theconjugated image width b_(k) (or a given image width b_(k) to theconjugated object width g_(k)) approximately as follows.

    D=1/g.sub.k +1/b.sub.k

wherein D is the nominal power of the lens. This approximation isrelevant for thin lenses, in which the two main planes practicallycoincide. Further, these conditions are applicable only for lenses inair or in a vacuum. If the conjugated values g_(k) and b_(k) are used inthe condition for the effective power D_(eff), for thin lenses in avacuum only, the nominal power is obtained. In other cases therelationship given above (D_(eff) =1/g+1/b) represents a definition ofan "effective" power D_(eff).

If a medium with the refractive index n_(v) is located in front of thelens, and behind the lens a medium with the refractive index n_(h), theconjugated object or image widths can be determined with the aid ofcalculation of the ray path through the lens. With this, the deflectionsof the rays on the refracting lens surfaces are calculated by means ofSnell's law of refraction. In the case of spherically curved lenssurfaces, and small lens apertures, the basic relationship

    n.sub.1 /a+n.sub.2 /b=(n.sub.2 -n.sub.1)/r

can be used, wherein n₁ is the refractive index in front, and n₂ is therefractive index at the rear of the refracting surface with thespherical radius r, and wherein a and b are the distances measured infront of and behind the refracting surface along a normal on therefracting surface. In each case, the conjugated object and image widthscan be determined for a light ray emitted from an object point; further,with the aid of considerations set out here--and known--a lens or a lenszone can be designed so that all light rays originating from the objectpoint are broken at the same conjugated image point, see below.

The dependency of ΔP of the distribution of the effective power upon thelens aperture A (and the wavelength) is represented in FIG. 3. It isnoted that this half-value width is independent of the nominal power ofthe lens.

FIG. 4 can be derived from the results of FIG. 3. As can be seen, thehalf-value width ΔP of the power distribution can be given as a goodapproximation of the function

    ΔP=λ*0.0056/F

shown in FIG. 4 by a broken line, wherein ΔP is the half-value width indioptres, λ is the wavelength in nm and F is the radiating lens surfacein mm².

The reduction of the half-value width with an increasing lens surfacecan be seen from the results in FIG. 5, in which the intensity I of thelight normalised on the lens surface F is plotted with respect to thiseffective power D_(eff) for a lens with a nominal power of fourdioptres. The lens with an aperture A of 3.16=√10 mm has a surface tentimes larger than one with a diameter of 1 mm, the half-value width (andthereby the depth of field) of the larger lens is correspondinglysmaller.

In order to understand the present invention, it is now essential thatthe interference pattern (7), depicted in FIG. 6 behind a circularradiating surface 5, is the interference pattern (8) behind an annularsurface 6 concentric to this circular surface, when the area of thecircular surface 5 and of the annular surface 6 is identical; thisresult can be derived, for example, directly from the theory of theFresnel zone plate.

To multiply the intensity, the circular surface is surrounded by as manyconcentric annular surfaces as desired. In order to obtain the opticalproperties of radiating surfaces with a small area, with a system usingsuch small surfaces the interference of the waves from the individualsmall surfaces has to be prevented. The interference of waves fromdifferent partial areas of a radiating surface differ by at least thecoherence length of the emitted (or re-emitted) light; the optical pathlengths are measured from the location of the light emission to thelocation of the interference or non-interference (image point orpotential image point).

As described in standard works (for example, Bergmann-Schafer, Optik[Optics] pps 331 ff, Max Born, Berlin-Heidelberg, New York 1972, p. 111)two light waves interfere with one another when the difference in theiroptical path length is smaller than the coherence length C.L.=λ² /Δλ,wherein λ is the average wavelength and Δλ is the half-value width ofthe wavelength distribution of the spectrum transmitted by the lightsource. The coherence length of "white light" is approximately 1 μm (seeBergman-Schafer, loc.cit p. 333); this value is obtained directly byusing λ=55 nm and Δλ=300 nm (white light comprises a wavelength range ofapproximately 400 to 700 nm).

In optical and ophthalmic applications, as a rule light re-emitted fromobjects is being dealt with. In FIG. 7 the re-emission spectra of bluecar paint 10, the leaf in a bunch of roses 11 and of a yellow apple areshown. The coherence lengths of these objects are 2 μm, 3.6 μm and 2.3μm. From these results it can be concluded that the coherence lengths ofthe visible light emitted from objects seldom exceeds values ofapproximately 5-10 μm (in comparison, laser light which is emitted in anextremely narrow wavelength range has coherence lengths of many meters).

In order, for example, to now configure the individual zones of a lensso that the light waves do not interfere behind the lens because of thedifferent zones, it is sufficient to configure the zones so that theoptical path lengths of the light rays associated with the light wavesthrough the different zones have differences of at least the coherencelength. Such zoned lenses or optical devices are hereinafter referred toas "coherence length corrected".

FIG. 8a shows a possible embodiment. The lens 15 is formed such thateach of its zones 16 has a nominally equal power (the nominal power isthen the power at which the distribution curve for the effective poweris maximum; see FIG. 2). The curvature of the front or rear surface ofthe zones 16 can be calculated according to known lens formulae.Further, the zones 16 can be configured so that the optical path lengthsof all the rays through a given zone 16, measured from an object pointpassing through the zone 16 to the conjugated image point correspondingto the nominal power (in the case of negative lenses or zones 16, theimage point is virtual), are exactly equal (the expression "aplanatic"is occasionally used for such lenses or lens zones). The individualzones 16 of the lens are now of different thicknesses, so at the bordersbetween the zones there are steps 17. The height of the individual steps17 between the zones 16 must now be at least C.L./(n₁ -n_(i)), whereinn_(i) is the refractive index of the medium adjacent to the lens 15 andn₁ is the refractive index of the lens material, so that the differencesin the optical path lengths of the waves associated with the raysthrough the different zones 16 are greater than the coherence length ofthe light processed. If the axis of the lens represents, for example,the z-axis of a system of coordinates, the height of the steps 17 is theabsolute amount of the difference between the largest z-coordinate ofthe front or rear surface of the one zone 16 and the smallestz-coordinate of the front or rear surface of the zone 16 bordering thiszone 16.

The steps can, as already described, naturally be configured either onthe front surface or the rear surface of the lens, and embodiments inwhich both lens surfaces are provided with steps are also possible. Inorder to avoid scattered light from the lateral surfaces of the steps17, they can be covered with a light absorbing material. The stepsbetween adjacent zones can also be inclined and/or curved, so adjacentzones are connected not by cylindrical walled surfaces, but instead byconical walls or barrel-shaped walls. Such wall surfaces can themselvesbe considered as annular lenses with a smaller total surface and verylarge absolute refractive power. The (slight) amount of the lightincident upon the wall surfaces compared to the total amount of lightincident upon the lens is then very much broken by these wall surfaces,and occurs as background intensity. In this respect, these areas shouldnot be considered as "zones" of the zoned lens according to theinvention, however, but rather as transitional areas between adjacentzones. It can also be of use to cover such wall surfaces with a lightabsorbing layer.

A lens according to FIG. 8a can, for example be configured as a contactlens. In recent years methods have also been developed for correctingrefractive errors in the eye by appropriate ablation of the corneaitself by means of a laser (excimer laser). Because of the degree ofprecision, in principle, of removal of corneal layers by means of alaser, there is the possibility of making an appropriate coherencelength correction directly on the surface of the cornea. The refractiveindex n_(i) is then that of the cornea (approximately 1.37).

If steps on the front or rear surface of the lens are not desired, theoptical path difference desired can also be produced in the individuallens zones by the use of material with different refractive indices.FIG. 8b schematically shows such a lens: zones 19 (only two of these areshown in FIG. 8b) in which a material with a refractive index n_(g) isused alternate with zones 20, which have a material with a refractiveindex n_(k) (only one shown), wherein n_(g) >n_(k). For cases where allthe zones 19, 20 have the same nominal refractive power, the curvaturesof adjacent zones 19, 20 are different. In this way the theoreticalcentral thicknesses t₁, t₂, t₃, . . . belonging to the individual zonescan be constructed. A good approximation of the differences in theoptical path lengths for rays through the individual zones (measured forthe nominal refractive power) is given by

    ΔL.sub.12 -t.sub.1 *(n.sub.g -1)-t.sub.2 *(n.sub.k -1)

wherein ΔL₁₂ is the optical wave length difference between the raysthrough the zone 1 and through the zone 2.

In an analogue manner

    ΔL.sub.13 -t.sub.1 *(n.sub.g -1)-t.sub.3 *(n.sub.g -1)

is obtained.

In general, the difference ΔL_(1m) in the optical path lengths between azone 1 and a zone m is given by

    ΔL.sub.1m -t.sub.1 *(n.sub.1 -1)-t.sub.m *(n.sub.m -1)

wherein t₁ and t_(m) are the central thicknesses belonging to zones 1and m, and n₁ and n_(m) are the refractive indices in zones 1 and m.From this condition it can be inferred that it is possible, byappropriate selection of parameters (refractive indices, lensthickness), to make the difference in the optical path length of raysthrough any two zones greater than the coherence length of the lightused.

In the case, for example, of contact lenses, a step-shaped surface canlead to a loss of wearing comfort. A correction of coherence length is,however, also possible with lenses having both surfaces smooth, that isto say constant. FIG. 9 shows a possible embodiment. The step heights 21in this embodiment are now at least C.L./(n₁ -n₂) in size, wherein C.L.is the largest coherence length of the light to be dealt with, and n₁and n₂ are the refractive indices of the two lens materials 22, 23. Theindividual lens zones again have the same nominal refractive power.

A further possible embodiment, for example of a contact lens 24, isshown in FIG. 10. In this case the steps 25 on the rear surface 24a ofthe lens 24 are configured so that approximately 50% of the rear surface24a can be fitted to to the cornea 26, by which means good wearingcomfort can be obtained. Half of the zones 28 have indentations whichfill with tear fluid 27. It is now possible to configure a front surface24b differentiable constantly and in sections--that is to saysubstantially smooth--so that the differences in the optical pathlengths of two rays passing through different zones 28 is at leastgreater than the coherence length of the light concerned. Thispossibility is based on the following (FIG. 11); when a surface, forexample the rear surface 30 of a lens or lens zone is given (the rearsurface is rotationally symmetrical but not necessarily spherical), witha given initial thickness t, points P1, P2, P3 . . . on the frontsurface 31 can be calculated so that lens or lens zone has a uniformrefractive power, that is to say is "aplanatic". The connection of allP_(i) 's then represents the front surface 31. The rays 34 are thentransmitted from an object point 32 and go to an image point 33.Analogue considerations apply in the case of the front surfacepreviously described, wherein, in the case of the lens according to FIG.10, the influence of the tear fluid zoned lens has to be taken intoconsideration.

FIG. 12 shows the intensity I of the light dependent upon the effectiverefractive power D_(eff), wherein in the curve 35 the development of theintensity of a lens divided into 10 coherence length corrected zoneswith an aperture of 3.16 mm is shown. As can be seen, with 10 times thelight capacity allowed through, such a lens has an intensitydistribution of the effective power which corresponds to that of a lensten times smaller with only one zone and one aperture of 1 mm (curve36). If the zones of the lens are not coherence length corrected, thereis, by contrast, a distribution of the effective refractive poweraccording to FIG. 5.

With respect to the intensity accompanying the individual effectivepowers, it should be noted that the coherence length correction can notlead to an increase in the total light capacity allowed to pass throughthe lens, that is to say regardless of the zone structure the same lightcapacity is measured integrally directly behind the lens. Theconfiguration of the zones consequently influences the localdistribution of the total light capacity, but not the transmitted lightcapacity itself.

The requirements for the production of such lenses are somewhat lessthan for the production of conventional diffraction lenses (see, forexample, U.S. Pat. No. 4,340,283, U.S. Pat. No. 4,637,697). This isbecause while with diffraction lenses the height of the steps must beconfigured at around precisely 1/10 wavelengths or approximately 50 nm(see, for example, Stanley A. Klein and Zhuo-Yan Ho, "Multizone BifocalContact Lens Design" SPIE, Vol. 679, p. 25, August 1986), it is simplynecessary with coherence length corrected zoned lenses that the steps donot exceed a certain minimum value (several μm).

As can be seen, a lens according to FIG. 12 has a half-value width ofintensity distribution of approximately 4 dioptres. Such a lens istherefore suitable, for example, for correcting the hyperopia ofgeriatric vision with a larger requirement for distance addition.Because of the intensity development of this lens, this lens works bestfor the middle distance. As described already, the half-value width ofthe intensity distribution is independent of the nominal refractivepower of the lenses, with the result that such lenses can be used foremmetropia, hyperopia and myopia.

If the preference towards the middle distance with a coherence lengthcorrected lens is not desired, there is the possibility to provide thezones of such a lens with, for example, alternating different nominalpowers. FIG. 13a shows the dependency of the intensity I of the lightupon the effective power D_(eff) with such a bifocal lens, in which eachof the 5 zones (area of each being π/4 mm²) is provided with a nominalpower of 2 or 6 dioptres. A lens of this type provides, for example, anapproximately constant intensity of approximately 1 to 7 dioptres. If,in comparison, the zones of this lens are not coherence lengthcorrected, this results in the distribution of the effective power shownin FIG. 13b. Such a lens would therefore be trifocal, wherein theaverage power depends on the interference of light waves from thedifferent zones. A similar result was obtained by Klein and Ho (SPIE,loc.cit) for a bifocal refractive bi-zone lens. It should be noted thatso-called refractive bifocal lenses, for example according to U.S. Pat.No. 5,106,180 and PCT/JP92/01730 do not represent coherence lengthcorrected lenses, in which interference between zones with differentpowers can be observed.

The zones of a lens with alternating different powers can, as canimmediately be seen, be made from material with the identical refractiveindex or from different materials with different refractive indices. Ifthe zones are provided with different refractive indices, it is possibleto make both lens surfaces constant or smooth, as can be inferred fromthe above description (FIG. 8). If only a single optical material isused for the lens, steps have to be provided between adjacent zones; asthe step heights are generally only a few micrometers, it is alsopossible with stepped lenses to make the stepped surface approximatelysmooth, that is to say making transitional areas instead of the steps.Such transitional areas are practically inevitable in the production ofsuch lenses, for example by means of turning, as the turning tools donot have an infinitely small radius.

A further possibility for dealing with the distribution of the effectivepower is in that a coherence length corrected lens is made frombirefringent material. FIG. 14 shows the dependency of the intensity Iof the light upon the effective power D_(eff) of a lens with a 4.5 mmdiameter, which is divided in to 10 zones with the same area, whereinthe zones are corrected for coherence length. The lens is made from abirefringent material, the refractive index of which is 1.51 (forordinary rays) and 1.66 (for extraordinary rays). Curve 40 shows theintensity of the extraordinary rays, curve 41 shows the intensity of theordinary rays, and curve 42 shows the total intensity. The individualzones do not interfere as they are coherence length corrected, theordinary and the extraordinary rays do not interfere as in a knownmanner waves polarised orthogonally with respect to each other do notinterfere (see, for example, Max Born, loc.cit, p. 113). Such a lenscan, for example be used for the correction of myopia of approximately-7D distance addition in geriatric vision. It is known that by using apolarisation filter in combination with birefringent bifocal lenses oneor other of the powers can be suppressed (see, for example, U.S. Pat.No. 5,142,411).

It is known that an astigmatic eye can see in focus in a large distancerange, if looking through a hole with a small aperture. The disadvantageof such a vision aid is mainly in the low light intensity passingthrough the hole. As was described, this light intensity can besignificantly increased in that coherence length corrected zoned lensesare used instead of the hole. This zoned "lens" can clearly also beprovided with zero nominal power.

If losses in contrast or in the resolution capacity are not desired forsuch a correction of astigmatism, zones with a larger surface can beused, which are not configured in an annular manner but instead as"ellipse rings". While with annular zones the two radii of the m-th zoneadjacent to the zone are expressed by r_(m) -1=const.*√(m-1) and r_(m)-const.*√m, an analogue law of formation for the two axes of theellipses applies for ellipse-shaped zones. If the main axes of theellipses are, for example, vertical, the lens is provided in thehorizontal direction with a wider distribution of the effective powerthan in the vertical direction (see, for example, Max Born, loc.cit, p.161); consequently a corresponding astigmatism can be correctedcylindrically to a few dioptres. A lens with elliptical diffractionzones but without coherence length correction is proposed for correctionof astigmatism in U.S. Pat. No. 5,016,977.

A "zoned lens" 45 with a zero power is shown schematically in FIG. 15.In the example shown the delimitation surfaces (45a, 45b) are planar,whereby this device is designated as a stepped plate. Zones withsubstantially equal front and rear curvature radii can have an analogueconfiguration. For simplicity, such a configuration is also designatedas a "stepped plate".

It can immediately be inferred from what has been described previouslythat a further embodiment of such a stepped plate can be in that opticalmaterials which have different refractive indices are used in theindividual zones.

A stepped plate of the type described is mainly used to alter theoptical path lengths of the waves or rays passing through the individualzones so that these waves can no longer interfere. When such a steppedplate 45 is used in combination with a conventional lens 46 (see FIG.16) the optical behaviour of the combination corresponds to that of thecoherence length corrected stepped lens described above with only anominal power. Such a system has a greater half-value width ofdistribution 47 of the effective power than the correspondingdistribution 48 of the conventional lens 46 by itself.

In principle, the possibility arises for fitting a stepped plate in theray path of optical apparatuses, possibly temporarily, when, forexample, the depth of field of the apparatus has to be increased, forexample with a microscope in order to find the object more rapidly.

A further embodiment of a stepped plate 49 with zero nominal power isshown schematically in FIG. 17. In this case the individual zones 50have, for example, a hexagonal cross-section. The zones are configuredso that each height 51 of each zone 50 differs by at least C.L./(n_(z)-n_(u)) from the height of any other zones 50. In this case C.L. is thecoherence length of the light used, n_(z) is the refractive index of thelens material and n_(u) is the refractive index of the medium adjacentto the lens. In this way it is again guaranteed that the light wavespassing through the different zones 50 do not interfere.

If a stepped plate according to FIG. 17 is used in combination with aconventional lens, a substantially narrower distribution curve for theeffective power of such a combination is obtained than with analogue useof a stepped plate according to FIG. 15, when as previously theintensity distribution along the axis of the lens is considered. If, onthe other hand, the intensity distribution is considered along theconnecting lines between the focal point corresponding to the nominalpower and the central point of the zone, when light is incident parallelto the axis of the lens an intensity distribution is again obtained, thehalf-vaiue width of which corresponds to the surface of the zone (seeFIG. 4). In any case, such an arrangement can also contribute, forexample, to increasing the depth of field of the lens or of a lenssystem or optical apparatus.

It is proposed that devices for converting coherent light intoincoherent or non-interfering light, which are called stepped plateshereinabove, can have zone cross-sections other than circular, annularor hexagonal.

Naturally, it is possible to combine a stepped plate according to FIG.17 and a refractive lens in one piece; the methods described above arethen used in the design of the surfaces of the zones. FIG. 18schematically shows a lens 52 which has coherence length corrected zoneswith the same cross-section shape.

In the case of the coherence length corrected zoned lenses describedabove, with concentric annular zones, it is assumed that the zonesurfaces have the same area (Fresnel zone shapes). It is also possible,however, to use zones with an area increasing from the inside to theoutside. Then, with an increasing aperture of the lens or of the pupilsize, the depth of focus reduces with simultaneous increase in contrast.If the individual zones are configured so that they have alternately twodifferent nominal powers, with an increasing aperture the lens isincreasingly bifocal with increasingly clear powers delimited from oneanother. Theoretically, the reverse behaviour can also be obtained inthat the areas of the zones are made smaller from the inside to theoutside.

As mentioned, with conventional refractive bifocal zoned lenses there isinterference between zones with different nominal powers. If suchinterference is undesired, there is the possibility to isolate the zoneswith respect to interference, in that steps are provided between theadjacent zones. FIG. 19 shows a possible embodiment. Such a zoned lens55 is provided with two types of zones 56, 56', which differ from oneanother in their nominal power. The lens 55 is also constructed so thatthe optical wave paths through the zones (56 and 56') with the samenominal power for the rays from the object point to the correspondingconjugated image point (B₁ or B₂) are all equal. In order to now avoidFresnel interference if possible, it is appropriate to make the areas ofthe zones with the same power unequal. Such lenses then representapproximately actual "refractive" bifocal zoned lenses, that is to saythe interference between zones with different refractive power issuppressed.

It is known that Fresnel zone plates can be used for focusingelectromagnetic waves (in the first order of interference). Withconventional Fresnel zone plates the zones are alternately transparentand opaque, which results in a 50% loss in light intensity (in the orderof interference concerned). FIG. 20 shows a modification according tothe invention of the Fresnel zone plate, in which such a loss of lightdoes not occur. The zones 61, 61' of this zoned lens 60 are configuredso that the zones 61, 61' are alternately provided with the thicknessesd_(g) and d_(u). The thicknesses are selected so that(du-dg)*(n1-n2)>C.L. applies, wherein C.L. is the coherence length ofthe electromagnetic ray to be focused, n₁ is the refractive index of thelens material, and n₂ is the refractive index of the surroundings. Thetwo types of zones (61, 61') then represent mutually independent Fresnelzone plates. The light yield of such a zoned plated 60 is then increasedcompared to a conventional zoned plate by the factor of 2.

The individual zones of such modified zoned plates can now also beprovided with different nominal powers, whereby the zoned plate or zonedlens will be bifocal. In a further development of the concept, such alens could also be made multifocal. In the case of bifocal zoned lensesof the type described, a principal intensity is directed in thenullified order of interference, and in the ±1., ±2., . . . orderfurther light with strongly decreasing intensity.

It can directly be inferred from what has previously been described,that different optical materials can be used in the individual zonesinstead of a single optical material. The configuration of the steps ofone of the surfaces of the zoned plate can then be omitted. Materialswith different refractive indices can also--as is the case with all thelenses described up to this point--be combined with stepped zones.

In summary, it can be said that the zoned plates or zoned lenses madefrom isotropic optical materials with concentric annular zones can bedivided as follows:

Criterion I: Geometrical Proportions of the Zoned Surfaces.

I.1 Fresnel zones: The zones have the same size of area.

I.2 Any area for the individual zones.

Criterion II: Type of Coherence Length Correction:

II.A No correction (=conventional zoned lenses)

II.B Same optical path lengths in zones with the same nominal power

II.C Optical path lengths in different zones differ by at least thecoherence length.

Criterion III: Nominal Powers of the Zones

III.a All zones with the same power

III.b Different powers in different zones

The alternatives in the three criteria can easily be combined with eachother in order to obtain specific optical devices. If in criterion II,option A is selected, conventional lenses are obtained, the options II.Band II.C according to the invention, on the other hand, produce novellenses. The combination I.1*II.C*III.a represents a lens whichcorresponds to FIG. 12; conventional refractive zones, for example U.S.Pat. No. 5,106,180, U.S. Pat. No. 4,704,016, U.S. Pat. No. 4,795,462 orPCT/JP92/01730 correspond to the combination I.2*II.A*III.b.

In FIG. 21 the distributions of the effective power of different bifocallenses is compared. FIG. 21a shows the distribution of a conventionalbifocal zoned lens, hereinafter known as lens A, in which all the zonesare 0.33 mm wide, FIG. 21b shows the distribution of a bifocal zonedlens according to the invention (lens B) in which interference onlyoccurs between zones with the same power, FIG. 21c shows thecorresponding distribution when all the zones are independent (coherencelength corrected) (lens C). The corresponding distributions for zoneswith an equal area in the lens are shown in FIGS. 21d-f, wherein FIG.21d is again a conventional lens (lens D) with constant transitionsbetween the zones and FIGS. 21e and 21f correspond to lenses accordingto the invention, wherein in FIG. 21e (lens E) interference occurs onlybetween zones with the same power and in FIG. 21f (lens F) there is nointerference between the zones. As can be seen, the conventionalrefractive zoned lenses (lenses A and D) are the basis for the coherencelength corrected embodiments (lenses B, C, E and F). The very wideintensity distributions of lenses C and F can be ascribed to the factthat the zones which are independent of one another have very smallsurfaces. If more contrast is desired, the number of zones can bereduced or the surface of the zones increased. Of particular note is theabsence of the power of -5D in lens A compared to lens B. This can beexplained immediately by comparing the optical path lengths (for theimage point for the power -5D) of the rays passing through theindividual zones (see FIG. 22): the difference in the optical pathlengths between all the rays through the lens A (curve 65) is just 2.8μm, whereby different--clearly mainly destructive--interference occursin the image point of the power of -5D. With the coherence lengthcorrected lens B (curve 66) there is exclusively constructiveinterference of all the rays in the zones with -5D power, the rays fromthe other zones with -2.5D have optical path lengths greater by at least10 μm, and thus do not interfere with the rays from the zones with -5D.

Reference is made expressly to the fact that with all coherence lengthcorrected refractive zoned lenses the constructive interference in themaxima of the effective powers is always in the nullified order; forthis reason such lenses have practically no chromatic aberration, as isthe case with diffractive bifocal lenses. Reference is also made to thefact that conventional refractive zoned lenses also have a veryconsiderable dependency upon the power distribution of the wavelengthsof the light, as there is always wavelength-dependent interferencebetween the individual zones (see FIG. 22).

In the previous embodiments it has always been assumed that the nominalpowers of the individual zones are refractive powers, that is to saythat these powers can be determined using the methods of geometricaloptics. The present invention also extends, however, to zoned lenses inwhich the individual zones coherence length corrected with respect toone another have diffractive powers. FIG. 23 schematically shows such acoherence length corrected diffractive lens 75. As can be seen, theindividual zones 76 are provided with diffractive sub-zones (76', 76",76'"), the differences in the optical path lengths of the light raysthrough the sub-zones 76', 76", 76'" have fixed relationships in a knownmanner. Between the optical path lengths of the light rays through thedifferent zones 76 the coherence length correction conditions accordingto the invention apply, however. Such a coherence length correction isthen advantageous when, for example, the depth of field of the twopowers of a diffraction lens has to be increased, that is to say whenthe intensity distribution has to be widened in the two powers. Further,for manufacturing reasons, it is sometimes difficult to implement therequired fixed phase conditions between diffractive zones which are farapart; errors in surface configuration of the order of 0.1 micrometerbetween the zones lying on the inside and outside can result inundesired destructive interference from light from such "untuned" zones.On the other hand, it is easier by comparison to obtain the requireddegree of precision for adjacent sub-zones 76, 76', 76". The object cantherefore be met by having to manufacture just the diffractive sub-zones76', 76", 76'" with the required precision and to isolate thediffractive zones 76 for interference reasons. With such lenses 76 thesums of the partial intensities of the individual zones is obtained inthe powers, and not the vector sums of the amplitudes--reduced in"untuned" zones.

To clarify and to summarise the invention, FIG. 24 shows the opticalpath length differences D in a conventional refractive bifocal zonedlens (nominal power 2.5 and 5.0 dioptres, which alternate zones have);this lens is thus not coherence length corrected. FIG. 24 shows the pathlength differences given for rays at different distances A from thecentre of the lens for different effective powers D_(eff). As can beseen, all the rays through a certain zone, when the nominal power ofthis zone corresponds to the effective power, have the same optical pathlength, wherein the path lengths or average values of these path lengthsthrough different zones are different. For an effective power whichrepresents the average value of the two nominal powers (for example,3.75 dioptres), the average value of the path lengths of the raysthrough all zones is the same. This explains the occurrence of anintensity maximum observed between the two nominal powers (see FIG.13b). For comparison, FIG. 25 shows the result for an embodiment of acoherence length corrected refractive zoned lens with the same nominalpowers at the effective power of 3.75 dioptres. With this lens betweenthe individual zones there are, for example, steps which cause thedifference in the optical path lengths between two rays passing throughdifferent zones in the immediate surroundings of the common zonedelimitation to be approximately 10 micrometers. As can be seen in FIG.25, with coherence lengths of under approximately 10 micrometers nointerference of light waves from the different zones can occur, whichmeans that for this effective power there is a summation of the scalarintensity from the individual zones (see FIG. 13a). Naturally thedifferences in the optical path lengths of rays passing through twoadjacent zones are, also for other effective powers in the case of thelenses presently described, about 10 micrometers, however forcharacterising the relationships, the conditions for the effective powerwhich corresponds to the average value of two adjacent zones withdifferent (or the same) nominal power is suitable.

The present embodiments deal with the conditions for electromagneticrays in the visible range. Naturally, analogous considerations can alsobe used for devices for dealing with wave-shaped rays of other typesand/or other wave length ranges.

The considerations described in the previous embodiments with respect tothe coherence length correction of zoned lenses can naturally also applyto the behaviour of mirrors and zoned mirrors for visibleelectromagnetic radiation, or also for electromagnetic radiation withother wave length ranges. The imaging equations of lenses can betransferred simply to the imaging conditions of mirrors, see for exampleBergmann-Schafer, Lehrbuch der Experimentalphysik, Vol. 3, Optik [Manualof Experimental Physics, Vol. 3, Optics] Berlin, New York 1993, page 88.Through this it is easily possible for the man skilled in the art to usethe above-described considerations and conditions for the conditions ofimaging mirrors or mirror systems. For this reason imaging mirrorsystems in which a coherence length correction is carried out, are alsodevices according to the invention, even though they are not discussedin full.

As an example, it is known that a parabolic mirror focuseselectromagnetic radiation incident parallel to the axis of the mirror atthe focal point. It may now be desired that a mirror focuses theincident radiation in different focal points. This can be achieved whenthe mirror is composed from different zones with different focallengths. If such zoned mirrors are configured so that their surface isconstant, disturbing interference (for example destructive interference)of radiation from the individual zones of the mirror can occur in theindividual focal points. If, on the other hand, the zones of amultifocal mirror are configured so that steps are located betweenadjacent zones, and so that the waves arriving from individual zones atthe focal point have differences in their path lengths of at least thecoherence length of the incident radiation, such possibly disturbinginstances of interference are suppressed.

If radiation is used which for which certain materials are transparent,that is to say that a refractive index can be given for such materials,mirror zones--analogous to the considerations set out above--can also becovered with materials with different refractive indices in order toobtain an appropriate coherence length correction.

I claim:
 1. Zoned lens with several zones, comprising a central circularzone and several circular ring zones concentrically arranged around saidcentral circular zone, wherein at least two adjacent zones areconfigured such that two light rays which pass through these twoadjacent zones have optical path lengths between an object point and animage point which are different by at least a coherence length of thelight used, which is at least 1 μm, and wherein further for obtaining adistribution of the effective power with a half-value width ΔP of atleast one dioptre the surface area measured in mm² of each zone has amaximum value of 0.0056λ, wherein λ is a wavelength of the light usedmeasured in nanometer.
 2. Zoned lens according to claim 1, wherein anytwo light rays which pass through these adjacent zones have optical pathlengths between the object point and each of the associated image pointswhich are different by at least the coherence length of the light used.3. Zoned lens according to claim 1, wherein the optical path lengthdifferences of all light rays which pass through two different zones ofthe lens are at least equal to the coherence length of the light used.4. Zoned lens according to claim 1, wherein when there is parallelincidence of the light, the differences in the optical path lengthsbetween a plane perpendicular to the direction of incidence and anassociated focal point of light rays which pass through two adjacentzones of the lens are at least equal to the coherence length of thelight used.
 5. Zoned lens according to claim 1, wherein the light raysparallel to the axis pass through optical path lengths inside the lensin two adjacent zones, the differences of which are at least three timesthe coherence length of the light used.
 6. Zoned lens according to claim1, wherein adjacent zones of the lens have a different thickness of lensmaterial, and wherein steps are provided between the zones.
 7. Zonedlens according to claim 1, wherein the step height between adjacentzones of the lens is at least |λ² /(Δλ(n_(c) -n_(i)))|, wherein λ is theaverage wavelength of the light used, Δλ is the half-value width of thewavelength distribution of the light used, n_(c) is the refractive indexof the lens material, and n_(i) is the refractive index of the mediumadjacent to the lens.
 8. Zoned lens according to claim 6, wherein whenvisible light is used with a coherence length in the region ofapproximately 1-10 μm, the step height, measured in micrometers, betweenadjacent zones of the lens is at least 5/|(n_(c) -n_(i))|, wherein n_(c)is the refractive index of the lens material, and n_(i) is therefractive index of the medium adjacent to the lens.
 9. Zoned lensaccording to claim 6, wherein the step height between adjacent zones ofthe lens is at least three micrometers.
 10. Zoned lens according toclaim 6, wherein the lateral surfaces of steps between the zones arearranged to be light-absorbent.
 11. Zoned lens according to claim 1,wherein adjacent zones of the lens have materials with differentrefractive indices.
 12. Zoned lens according to claim 1, wherein thelens has at least two layers of different materials, wherein at leastone interface between adjacent zones of the lens located between twosuch layers has steps.
 13. Zoned lens according to claim 12, whereinadjacent zones of the lens have a different thickness of lens material,and wherein steps are provided between the zones, and further whereinthe step height, measured in micrometers, between adjacent zones of thelens is at least 5/|(n₁ -n₂)|, when light is used with a coherencelength in the range of approximately 1-10 μm, wherein n₁ and n₂ are therefractive indices of the materials of the two layers.
 14. Zoned lensaccording to claim 1, wherein the areas of the zones are ofsubstantially equal sizes.
 15. Zoned lens according to claim 1, whereinthe refractive nominal power of the individual zones is equal.
 16. Zonedlens according to claim 1, wherein at least two types of zones areprovided, wherein all the zones of one type have the same nominal power,and zones of different types have different nominal powers.
 17. Zonedlens according to claim 16, wherein zones of different types aredirectly adjacent.
 18. Zoned lens according to claim 17, wherein for onetype of zones the optical path lengths of all light rays from the objectpoint through one of the zones of this type to a conjugated image pointbelonging to this type are equal.
 19. Zoned lens according to claim 16,wherein the differences in the optical path lengths of two light rayspassing through different types of zones of the lens from an objectpoint to an image point are at least equal to the coherence length ofthe light used.
 20. Zoned lens according to claim 1, wherein the opticalmedium of the lens is a birefringent optical material.
 21. Zoned lensaccording to claim 1, wherein at least a part of the zones havediffractive sub-zones, wherein these zones have diffractive powers. 22.Zoned lens according to claim 1, wherein for obtaining a distribution ofthe effective power with a half-value width ΔP of at least two dioptresthe surface area measured in mm² of each zone has a maximum value of0.0028. λ, wherein λ is a wavelength of the light used measured innanometers.
 23. Zoned lens according to claim 1, wherein for obtaining adistribution of the effective power with a half-value width ΔP of atleast four dioptres the surface area measured in mm² of each zone has amaximum value of 0.0014. λ, wherein λ is a wavelength of the light usedmeasured in nanometers.
 24. Zoned lens according to claim 1, wherein thelight used is visible light.
 25. Zoned lens according to claim 1,wherein the light used is infrared light.
 26. Zoned lens according toclaim 1, wherein the light used is ultraviolet light.
 27. Zoned lensaccording to claim 1, wherein the lens is an ophthalmic lens.
 28. Zonedlens according to claim 1, wherein the lens is an optical instrument forcorrection of astigmatism regardless of the axis, in particular ofirregular astigmatism.
 29. Zoned lens according to claim 1, wherein thelens is an intracorneal lens.
 30. Zoned lens according to claim 1,wherein the lens is a contact lens.
 31. Zoned lens according to claim 1,wherein the lens is a spectacle lens or part thereof.
 32. Zoned lensaccording to claim 1, wherein the lens is an intraocular lens.
 33. Zonedlens according to claim 1, wherein the lens is an optical system,preferably a telescope or microscope.
 34. Zoned mirror, which includesat least two adjacent zones which are configured so that the differencesin the path lengths between an object point and an image point ofradiation which is incident upon these two zones are at least equal tothe coherence length of the radiation used.
 35. Zoned mirror accordingto claim 34, wherein for parallel incidence of radiation, differences inthe path lengths of rays which are reflected from different zones, whichdifferences occur between a plane perpendicular to the direction ofincidence and a focal point, are at least equal to the coherence lengthof the radiation used.
 36. Zoned mirror according to claim 34, whereinfor obtaining a distribution of the effective power with a half-valuewidth ΔP of at least one dioptre the surface area measured in mm² ofeach zone has a value of maximal 0.0056. λ, wherein λ is a wavelength ofthe radiation used measured in nanometers.
 37. Zoned mirror according toclaim 34, wherein the coherence length of the light used is at least 1μm.
 38. Zoned mirror according to claim 34, wherein the light used isvisible light or infrared light or ultraviolet light.
 39. A zoned lenscomprising at least two types of zones, wherein all zones of one typehave the same nominal power, and zones of different types havingdifferent nominal powers and are direction adjacent, wherein at leasttwo adjacent zones are configured such that two light rays which passthrough these two adjacent zones have optical path lengths between anobject point and an image point which are different by at least acoherence length of the light used, which is at least 1 μm, and whereinfor one type of zone the optical path lengths of all light rays from theobject point through one of the zones of this type to a conjugated imagepoint belonging to this type are equal.